This study was initiated to determine the structure:function relationship of RNA polymerase II of Drosophila melanogaster. This enzyme is a heteromultimer consisting of approximately ten different subunits, each of which is presumably specified by a different locus. To date one locus, a mutant of which confers alpha-amanitin resistance, has been identified. This locus will be used as a toe-hold to identify the loci coding for the other subunits by means of genetic interactions between this and the other loci.